In recent years the automobile industry has engaged in intensive research to develop coatings which require reduced energy consumption and which may be applied at reduced volatile organic compound (VOC) emissions levels. Recently developed multiple component sprayable coatings, which crosslink or cure upon combination of the components as they are applied, appear to offer a potential for not only reducing oven temperatures, and hence energy consumption, without adverse effect to the quality of the finish or the time required for completing crosslinking of the coating, but also potential for reducing VOC emissions. These multiple component coating materials which crosslink on the substrate after application normally include a crosslinking "accelerator" which is capable of increasing the rate of crosslinking at any given temperature. By varying the quantity of accelerator present in a coating, the crosslinking rate may be varied. The presence of any accelerator induces a tendency to crosslink at low, ambient, temperature and necessitates the maintaining of the material in component form until immediately prior to application as a coating.
Various multiple component coatings are available and will be apparent to those skilled in the art. One general class of two component paint system is based on polyurethane chemistry. Compositions of this type require one component containing a hydroxy functional resin (usually the color component) and another component containing an isocyanate crosslinker. These two components must be combined just prior to application to the substrate because the components would react prematurely if combined earlier. As is the case with all multiple component paint systems where a crosslinking reaction occurs after combination of components, the materials must be combined in precise amounts and, therefore, must be pass through or into the spraygun in metered amounts where they are combined just prior to application to the substrate. In order to combine the components in the proper ratio, it is therefore necessary to include metering means such as metering pumps and flow sensors as part of the apparatus used to apply the coatings.
Since multiple component materials must be combined in precise ratios as discussed above, thus necessitating more complex metering equipment, color change and flushing necessary to do so are greatly complicated. This is a particularly significant problem when coating operations require frequent, rapid color changes, such as in the automobile industry where it is very common to change colors between units on the line. In order to change a color, substantial quantities of solvent must be expended in order to remove all vestiges of coating material, particularly crosslinked material, from the spray apparatus and to remove all vestiges of the color component from those portions of the apparatus through which the color component passes. In order to avoid the necessity of having separate feed apparatus for each selected color including separate metering means for each, it is necessary to provide a means of rapidly flushing a single apparatus which can be used for all colors. It is also highly desirable to accomplish this flushing of the spray apparatus including the feed apparatus for the color component with a minimum amount of solvent and with as little waste of coating materials as possible. It is therefore a specific object of the present invention to provide an improved apparatus for effecting rapid color change of the color component of a multiple component sprayable fluid to be applied using a manual spraygun.
U.S. Pat. Nos. 3,145,930 to Herklotz et al. and 3,219,273 to Killen each disclose painting systems adapted to be flushed prior to changing colors. While neither of these painting systems employs multi-component paints, they do include pumps which must be flushed prior to color change. U.S. Pat. Nos.: 3,155,531 to Juvinall; 3,403,695 to Hopkins; 3,450,092 to Kock; 3,458,133 to Wiggins; 3,477,870 to Boretti et al.; 3,672,570 to Scarborough et al.; 3,674,205 to Kock; and 3,857,513 to Wiggins all teach painting or coating systems providing for color change.
U.S. patent application Ser. No. 260,221 filed May 1, 1981, now abandoned, in the name of Courtney et al. and assigned to Ford Motor Company, assignee of this application, teaches a flushable metering pump for use in a painting apparatus requiring rapid color change. This flushable metering pump, which is taught as being particularly suitable for use in systems for application of mutli-component paint compositions, is particularly well suited for use in the apparatus of the present invention. U.S. Pat. Nos. 3,924,806 and 4,005,825 to Schowiak and also assigned to Ford Motor Company, teach a spraygun including a mixing manifold, which is particularly suitable for use in spraying multi-component coating compositions wherein the components must be admixed just prior to application. The spraygun taught in these patents is particularly suitable for use in conjunction with the apparatus of the present invention.